I looked everywhere and found nothing so I created the following and thought I would share for everyone facing the same need.

I made this for X3 version 7

• • • Go to Development/Script Dictionary/Screens/Screens (GESAMK)
      • Search for screen code SCO2 "Order sub-contract materials"
      • Add reference table ITMMASTER on General Tab
    • 
    • Next Click on fields tab
      • Add field from table ITMMASTER field for description ( see below for screen shot )
        • Go to first empty line and use field lookup and add ITMDES1 Field from ITMMASTER table
        • Set block to "1"
        • Set Position to next available number
        • Change Description if wanted from default "Description 1" below was set to "Description"
        • Set input to "Display"
        • Set mode column to "Table"
        • Add activity code to protect new field from future validations
        • Click save
    • 

• • • Find field "CPNITMREF"
      • Click on field
      • Go to ACTIONS block
    • 
    • On blank row
    • Select from Type drop down "Before field" and tab
    • 
    • Click Save
      • This will create a Specific Processing file to be used in next step
    • Validate entire screen SCO2
  •  
    • Go to Development/Script Dictionary/Scripts/Script editor (ADOTRT)
    • Type SPESCO2 into File Name Box and tab off
    • In the window you should see below
    • Find Subprogram AV_CPNITMREF
    • 
    • Between Variable declaration and the word End add following 5 code lines 

                                 if clalev([F:ITM])=0 : Local File ITMMASTER[ITM] : Endif

                                 Read[F:ITM]ITM0=VALEUR

                                 if !fstat and VALEUR<>''

                                [M:SCO2]ITMDES1(nolign-1)=[F:ITM]ITMDES1 : Affzo [M:SCO2]ITMDES1

                                Endif

• • • Should look like this when complete
    • 
    • Click Compile
    • Close Page
    • Go to Setup/Purchasing/Entry transactions/Sub contract orders
    • From left list select one by one each entry transaction and then Click VALIDATE Button
    •  

• • • Finally Log out and log back in and check Subcontract Materials tab for description( will be last column in detail view until moved )
    • 

Thank you so much for the detail response. I will try it out.

I've tried this with no luck. Trying to do the same but adding to Product Sites to start. Am I missing something?

My Script - 

Subprog AV_ZLONGDESC(VALEUR)

Variable Char    VALEUR()

if clalev([F:ITM])=0 : Local File ITMMASTER[ITM] : Endif

Read[F:ITM]ITM0=VALEUR

if !fstat and VALEUR<>''

[M:ITF0]ZLONGDESC(nolign-1)=[F:ITM]ZLONGDESC : Affzo [M:ITF0]ZLONGDESC

Endif

I did not have the Action setup on the correct field for this to kick off. This is actually working great. Thanks!